Andrija Mohorovičić (23 January 1857 – 18 December 1936) was a Croatian meteorologist and seismologist. He is best known for the eponymous Mohorovičić discontinuity and is considered as one of the founders of modern seismology.[1]

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Mohorovičić was born in Volosko, Opatija, where his father (also named Andrija), was a blacksmith, making anchors. The younger Andrija also loved the sea and married a captain's daughter, Silvija Vernić. They had four sons. Mohorovičić obtained his elementary education in his home town, then continued at the gymnasium of neighbouring Rijeka. He received his higher education in mathematics and physics at the Faculty of Philosophy in Prague in 1875, where one of his professors was Ernst Mach. At 15, Mohorovičić knew Italian, English and French. Later he learned German, Latin and Ancient Greek.[2]

Andrija Mohorovičić (c. 1880).

Detail of a commemorative plaque of Andrija Mohorovičić in Clementinum, Prague, Czech Republic.

On 13 March 1892, he observed the tornado in Novska, which picked up a 13-ton railway carriage with fifty passengers and threw it 30 m. He observed also the "vihor" (whirlwind) near Čazma in 1898 and studied the climate in Zagreb. Mohorovičić was the first person to describe atmospheric rotors with a horizontal axis, which he observed during bora-wind episodes in the northern Adriatic.[4] In his last paper on meteorology (1901), he discussed the decrease in atmospheric temperature with height. His observations of clouds formed the basis of his doctoral thesis On the Observation of Clouds, the Daily and Annual Cloud Period in Bakar presented to the University of Zagreb and which earned him his degree as doctor of philosophy in 1893.[2][3]

On 8 October 1909 there was an earthquake with its epicentre in the Pokuplje region, 39 km southeast of Zagreb. A number of existing seismographs were installed before and provided invaluable data upon which he made new discoveries. He concluded that when seismic waves strike the boundary between different types of material, they are reflected and refracted, just as light is when striking a prism, and that when earthquakes occur, two waves—longitudinal and transverse—propagate through the soil with different velocities. By analyzing data from more observation posts, Mohorovičić concluded that the Earth has several layers above a core. He was the first to establish, based on the evidence from seismic waves, the discontinuity that separates the Earth's crust from its mantle. This is now called the Mohorovičić discontinuity or (because of the complexity of that name) Moho. According to Mohorovičić, a layered structure would explain the observation of depths where seismic waves change speed and the difference in chemical composition between rocks from the crust and those from the mantle. From the data, he estimated the thickness of the upper layer (crust) to be 54 km. We know today that the crust is 5–9 km below the ocean floor and 25–60 km below the continents, which are carried on tectonic plates. Subsequent study of the Earth's interior confirmed the existence of the discontinuity under all continents and oceans.

Mohorovičić assumed that the velocity of seismic waves increases with the depth. The function he proposed to calculate the velocity of seismic waves is called the Mohorovičić law.[5][6] He developed a method for determining earthquake epicenters[7] and constructed curves giving the travel times of seismic waves over distances of up to 10,000 miles from the source.[8][9] He also proposed the construction of a new type of seismograph for recording the ground horizontal movement, but due to lack of funds the project was never realized.[10]

As early as 1909 Mohorovičić started giving lectures that both architects and building contractors should follow, ahead of his time setting some of the basic principles of earthquake-resistant design.[2][11] Mohorovičić's theories were visionary and were only truly understood many years later from detailed observations of the effects of earthquakes on buildings, deep focus earthquakes, locating earthquake epicenters, Earth models, seismographs, harnessing the energy of the wind, hail defence and other related elements of the geological body of knowledge known as geoscience.

A. Mohorovičić (1908). "Epicenters of earthquakes in Croatia and Slavonia (Epicentra potresa u Hrvatskoj i Slavoniji)". Yearly report of the Zagreb meteorological observatory for the year 1906. Part IV. pp. 15–19.

A. Mohorovičić (1910). "Epicenters of earthquakes in Croatia and Slavonia (Epicentra potresa u Hrvatskoj i Slavoniji)". Yearly report of the Zagreb meteorological observatory for the year 1909.